Adhesives

Adhesives unite materials, creating a whole that is greater than the sum of its parts . Their volume is small compared to the metals, glass, wood, paper, fibers, rubber, and plastics they bond.
The “adhesive” bonds “adherends,” which are substrates such as glass, metal, plastics, and wood . In a typical adhesive bond, the basic components are:
(SUBSTRATE/INTERFACE/ADHESIVE/INTERFACE/SUBSTRATE)
Adhesives may be classified in many ways including mode of application and setting, chemical composition, cost, suitability for various adherends, and end products. Adhesives must be applied to substrates in a fluid form to wet the surfaces, which requires low viscosity to flow onto the surfaces while eliminating voids. After application to surfaces (adherends), the adhesive must solidify to develop bonding strength. The transition from fluid to solid may be accomplished in the following ways:
1. Cooling of a thermoplastic. Thermoplastics soften and melt when heated, becoming hard again when cooled. Methods of applying adhesives in this way include hot-melt applicators; dry powders that are heated after application ;and extruders.
2. Release of solvent or carrier. Solutions and latices contain the adhesive composition in admixture with water or organic solvents. These liquids lower the viscosity to permit wetting ofthe substrate. After wetting has been accomplished, they must be removed.
3. Polymerization. The fluid adhesive is applied to the substrate followed by rapid polymerization to bond the substrates. The reaction-sensitive adhesives fall into two main groups: condensation and addition polymenzations.
4. Pressure-sensitive adhesives. These adhesives are fluid applied and do not undergo a chemical reaction. After wetting the substrates, they remain in the gel state which is a tackiness capable of being removed rather than a permanent bond.